Various methods are known in the prior art for assembling films. These methods allow longitudinally joining films with thicknesses from a few tens to several hundreds of microns, the junction being in some cases reinforced by a longitudinal reinforcing band. Among the known methods are such techniques as flat-welding, called "peeling", where the two films are placed one on the other, with the edges to be joined in alignment. A reinforcing strip, which may be planar or comprise a folded ribbon, furthermore may be assembled together with the two films.
Another known method comprises placing the films edge against edge and joining them by a flat-laid cover tape straddling both films. All these methods enable, with more or less difficulty, joining films with minimum thicknesses of about 15 microns in the case of polyethylene films. However these known methods do not permit joining thinner films and in particular, thin polyester films. Handling these very thin films and joining them are exceedingly delicate operations, in which the slightest onset of tearing could possibly result in total film degradation. All these methods, which require handling of the film at a time when it is still unprotected, therefore do not assure a high-grade joining or assembling of the films.
On the other hand there is significant interest in polyester films because of their high strengths. At a given thickness, a polyester film is ten times stronger than a polyethylene film. This fact takes on great significance in the field of space balloons where the optimal trade-off between strength and weight is sought in order to move the largest possible load to the greatest possible altitudes. For strength reasons, the use of polyethylene prevents use of too thin films (less than 15 microns) because their low circumferential strength could cause the ruining of such balloons in spite of the balloons assuming a natural shape without circumferential stresses--at least under static conditions. It has been observed in practice, however, that there are dynamic forces which create significant circumferential forces.
As a result, present-day balloons illustratively may carry a load 500 kg to an altitude of only 45 km, the dead weight of this envelope being about 1,000 kg. Under the same conditions, an ultra-thin polyester balloon with a dead weight of 350 kg might carry that same load to a 55 km altitude.
Accordingly a primary object of the present invention is to overcome the disadvantages of the prior art methods.
Another object of the invention is to provide a process for joining thin films made of a material such as polyester having a minimum thickness of between 3 and 5 microns, without a danger of starting a tear.
Still another object of the invention is to create an envelope of a space balloon which is made of 3 to 5 microns thick polyester.